Field of the Invention
The present invention relates to a gas sensor element for detecting a particular gas contained in gas to be measured, and a gas sensor having such a gas sensor element.
Description of Related Art
An example of a conventionally known gas sensor having a gas sensor element for detecting a particular gas contained in gas to be measured is an oxygen sensor installed in an exhaust flow path, such as an exhaust pipe of an internal combustion engine, and utilized in controlling combustion of the internal combustion engine through detection of the oxygen concentration of exhaust gas. Other examples of such a gas sensor include a NOx sensor for detecting NOx concentration and an air/fuel ratio sensor for detecting the ratio between air and fuel, etc.
Such a gas sensor has, for example, a tubular metallic shell, and a gas sensor element held in the metallic shell.
The gas sensor element may be a plate-shaped gas sensor element. The plate-shaped gas sensor element includes a plate-shaped solid electrolyte member having a pair of electrodes disposed on the surface thereof, and a dense member which is stacked on the solid electrolyte member and forms at least a portion of a wall surface of a hollow cavity into which gas to be measured or the atmosphere is introduced.
Notably, the gas sensor element is manufactured by firing a green (unfired) laminate formed of ceramic. The green laminate is formed by providing, by means of printing and laminating, on a green solid electrolyte sheet which is to become the solid electrolyte member, green electrodes which are formed primarily of metal and are to become the electrodes, a green wall surface sheet which is to become the wall surface of the hollow cavity, etc.
Such a gas sensor element has the following problem. At the time of manufacture, small damage (green breakage) may occur in the green solid electrolyte sheet due to volume shrinkage occurring as a result of heating in a debindering step or the difference in thermal shrinkage between ceramic and metal caused by a change in temperature at the beginning of a firing step. In some situations, due to a thermal shock, a portion where such a green breakage has occurred becomes a start point of cracking, and a crack is produced in a solid electrolyte member formed through firing. In such a case, the function of the gas sensor element is impaired.
In order to solve such a problem, there has been proposed a gas sensor element which restrains shrinkage of a green electrode to thereby restrain green breakage through employment of a structure in which an end portion of an electrode is held and covered by a dense member (Patent Document 1).